1. ABG AND ELECTROLYTE ABNORMALITIES ALEX BUTTFIELD

2. A DISCLAIMER

3. OUTLINE My approach to acid/base disturbances Multiple examples A few mnemonics Exam hints

4. MY APPROACH TO ABGs pH first (obviously) – Tailor approach to acidaemia vs alkalaemia and primary disturbance (respiratory vs metabolic)

5. MY APPROACH TO ABGs Metabolic acidosis: Anion gap – Na –(HCO3+Cl) – Raised if >12 – Normal 8-12 – Low <8 Expected CO2 (Winter’s formula  HCO3x1.5 + 8) Delta ratio (AG -12/24-HCO3) A-a gradient (not applicable if VBG or if FiO2 unknown) Electrolyte correction: – Na  Measured Na + (BSL/3 -10) – K  for every 0.1 pH below 7.4, subtract 0.5 from measured K Osmolality and osmolar gap (if applicable)

6. DIFFERENTIALS FOR HAGMA KETONES: – DKA – Starvation – Alcoholic ketoacidosis RENAL FAILURE LACTATE (type A and B) TOXINS (SEPTIC): – Salicylates – Ethylene glycol, methanol – Paraldehyde – Toluene – Iron, INH – Cyanide, Carbon monoxide

7. NON-ANION GAP METABOLIC ACIDOSIS U = Ureteric diversion S = Sigmoid fistula E = Excess saline D = Diarrhoea (or resolving DKA) C = Carbonic anhydrase inhibitors A = Addisons R = Renal Tubular Acidosis P = Pancreatic fistula

8. LOW ANION GAP In other words, excess unmeasured cations, or decreased anions: – Hypoalbuminaemia – Excess unmeasured cations (Mg, Ca, Li, myeloma)

9. Metabolic alkalosis: – Expected CO2 (0.7xHCO3 +20), can compensate to CO2 ~60 – Expected electrolyte disturbances (low K, Cl) – Good list of differentials

10. DIFFERENTIALS FOR METABOLIC ALKALOSIS C = Contraction L = Loop diuretics, licorice E = Endocrine (hyperaldosteronism, Bartter’s Conn’s, Cushings) V = Vomiting (including NG suction) E = Excess alkali (antacids, milk-alkali, dialysis) R = Refeeding R = Renal bicarbonate retention

11. Respiratory acidosis: – Expected HCO3: Acute = 24 + ([PCO2-40]/10) x 1 Chronic = 24 +([PCO2 -40]/10) x 4 – ?coexistent with other metabolic disturbance – Good list of differentials

12. RESPIRATORY ACIDOSIS ACUTE: – Airway obstruction (any cause) – Aspiration – Bronchospasm – CNS depression (drugs, CVA, bleed) – Muscle weakness (GBS, Duchenne’s) – Pulmonary disease CHRONIC: – Chronic lung disease – NM disorders – Obesity (obesity hypoventilation syndrome)

13. Respiratory alkalosis: – Expected HCO3: Acute (most common) = 24 – (40-PCO2/10) x 2 Chronic = 24 – (40-PCO2/10) x 5 – Put in context – Differentials  HYPERVENTILATION

14. RESPIRATORY ALKALOSIS CHAMPS – CNS disease (raised ICP) – Hypoxia (any cause) – Anxiety – Mechanical ventilation – Progesterone, pregnancy – Sepsis, salicylates (and other toxins)

15. CASE ONE 5 y.o. girl, previously well. Recent history of weight loss and 2-3 days of fever. Presented tachypnoeic, poor central perfusion, drowsy but easily rousable. – pH 6.89 – PCO2 26 – PO2 23 (VBG) – HCO3 5 – BE -28 – Lactate 1.92 – Na 136 – K 3.8 – Cl 97

16. CASE ONE Profound, life-threatening raised anion-gap metabolic acidosis with co-existent respiratory acidosis. Severe whole-body potassium depletion. Evidence of hypoperfusion. – Causes: DKA (first presentation) Toxins Renal failure Lactate (normal)

17. CASE TWO 21 y.o. female, unwell for one month with worsening cough/SOB. Brought in by partner as he “couldn’t wake her” – pH 6.94 – PO2 459 (FiO2 1) – PCO2 134 – HCO3 27 – BE -7 – Lactate 8.94 – K 5.2

18. CASE TWO Life-threatening type 2 respiratory failure with coexistent mild metabolic acidosis with markedly elevated lactate: – Likely bronchospasm Other causes – Causes elevated lactate: Type A Type B

19. CASE THREE 29 y.o. female with chronic LBP and analgesic abuse presents with 3 days of bilateral ankle and facial swelling after taking “lots of painkillers”. – pH 7.01 – PO2 33 (VBG) – PCO2 35 – HCO3 9 – BE -22 – Na 138 – K 3.8 – Cl 120 – BSL 4

20. CASE THREE Severe non-anion gap metabolic acidosis with severe whole-body depletion of potassium. – ?causes

21. CASE FOUR 61 y.o. female, BIBA with ?seizure. 5-6 days of vomiting, some diarrhoea. – pH 7.65 – PCO2 55 – PO2 80 – HCO3 60 – BE >+30 – K 2.3 – Cl 69 – Lactate 1.76

22. What about the seizure?

23. CASE FOUR Profound metabolic alkalosis with complete respiratory compensation – Likely due to vomiting illness ?underlying cause of vomiting Severe hypokalaemia with marked prolongation of QTc, risk of arrhythmia/sudden death

24. CASE FIVE 54 y.o. male. Unwell with flu-like illness for two weeks, has had back ache for one week, for which he has been using “a cream”. – pH 7.60 – PO2 181 (FiO2 40%) – PCO2 16 – HCO3 15 – BE -4 – Na 135 – K 2.9 – Cl 98 – Lactate 1.2

25. CASE FIVE Severe respiratory alkalosis with co-existent raised anion gap metabolic acidosis – Causes?

26. CASE FIVE Causes respiratory alkalosis  CHAMPS – CNS disease – Hypoxia of any cause – Anxiety – Mechanical over-ventilation – Progesterone/pregnancy – Sepsis, salicylates and other toxins (methylxanthines)

27. CASE SIX 43 y.o. male, three day history of epigastric pain and recurrent vomiting. Tachycardic (130), tachypnoeic (36) but normotensive. PMHx moderate daily alcohol intake. – pH 7.47 – PCO2 23 – PO2 28 (VBG) – BE -6 – HCO3 17 – Lactate 2.38 – Na 136 – K 2.9 – Cl 85 – BSL 7.2 – Ketones “HI”

28. CASE SIX Complex acid-base disturbance: – Raised anion gap acidosis – Intercurrent metabolic alkalosis (given delta ratio, hypochloraemia, hypokalaemia) – Coexistent respiratory alkalosis – Ketosis, euglycaemic – Any thoughts?